This invention relates generally to optical demultiplexers, and, more particularly, to a wavelength selective optical demultiplexer tuner where a predetermined wavelength is extracted or tuned from an input signal containing a plurality of wavelengths and this predetermined wavelength is received at a single preselected point on the demultiplexer tuner.
In practice, the need frequently arises for communications or reconnaissance systems which simultaneously convey multiple messages for a large number of information sources in one location to a large number of users at another location. Multiplexing systems economically meet this need by combining the messages from several information sources, which are then transmitted as a composite group over a single transmission facility, with a provision at the receiver for separation (demultiplexing) back to individual messages.
In recent years with the development and implementation of fiber optic technology into practical transmission systems a great deal of attention has been given to a multiple carrier technique referred to as wavelength division multiplexing (WDM). This technique, which is the optical equivalent of the frequency division multiplexing technique employed in RF coaxial transmission networks, can be used to increase the information transfer capacity of the medium. In the wavelength division multiplexing technique each discrete data channel is modulated onto an optical carrier of a fixed wavelength. Each of the individual carriers are then superimposed onto the optical transmission medium. At the optical receiver the individual carrier must be reestablished by filtering the composite carrier into its individual wavelength components.
Generally, when this composite wavelength multiplexed signal is received by the demultiplexer, a number of techniques have been developed which separate each of the multiplexed signals into a plurality of outputs and receive, from the demultiplexer at a plurality of separated points, each of the separated previously multiplexed signals. An example of such a prior optical demultiplexer can be found in U.S. Pat. No. 4,294,508 issued on Oct. 13, 1981 to this inventor. Another such demultiplexing technique involves the use of a Graded Index (GRIN) rod having a blazed grating associated with one end thereof. In such a technique, as illustrated, for example, in the following articles by Tomlinson et al entitled "Optical multiplexer for multimode fiber transmission systems" Applied Physics Letters, Vol. 31. No. 3, Aug. 1, 1977, pgs. 169-171, and "Optical wavelength-division multiplexer for the 1-1.4 .mu.m spectral region," Electronics Letters, Vol. 14, May 25, 1978, No. 11, pgs. 345-347, the input composite wavelength multiplexed signal has each of its individual wavelengths diffracted at a slightly different angle by the grating and appear as a series of displaced positions along a face of the GRIN rod. In such a technique, it can be seen that the demultiplexer acts as a series of bandpass filters, narrow-banding the optical input signal to a plurality of displaced positions.
One of the inherent problems associated with such systems is the physical placement of the output optical fibers on the front face of the demultiplexer. It is the physical placement of the optical receiving fibers which determine what wavelength is to be observed. As it is most difficult to procure lasers with unique wavelengths, the laser's wavelength must be carefully selected and perhaps thermally tuned to place it within the bandpass of the established demultiplexer channel. Furthermore, in many applications, such as local area networks and TV distribution systems, it is not necessary to extract all the wavelengths on the composite optical carrier simultaneously. Therefore, the utilization of such prior demultiplexing techniques as described above tend to be inherently inefficient and generally unacceptable for single wavelength demultiplexing applications.